In modern microelectronics, there are two main photodetector (PD) configurations, namely vertical photovoltaic type PDs such as p-type intrinsic n-type (PIN or PN) diodes and lateral photoconductive type PDs such as metal-semiconductor-metal (MSM) photoconductors.
In a photovoltaic-type detector, incident photons are absorbed in an active semiconductor layer sandwiched between two electrodes one of which should be transparent to the photons such as transparent conductive oxides. The photovoltaic-type detector works under the reverse bias conditions in which the created electric field sweeps the photogenerated carriers to form the photocurrent.
In a photoconductive-type detector, incident light is absorbed directly by the photoconductive layer and the photocurrent signal is obtained by applying an electric field across the photoconductive layer.
Generally, the photovoltaic-type detector has higher conversion efficiency and larger fill factor. However, the photoconductive-type detector is more advantageous in terms of integration simplicity, ease of fabrication, cost effectiveness and speed.
Typically, a lateral MSM PD includes Ohmic or Schottky contacts on a top or at a bottom of the semiconductor (e.g. amorphous silicon) layer which functions as a photoconductor instead of a diode. MSM PDs are biased such that one contact is forward biased and the other one is in the reverse bias. Upon light illumination, photons are absorbed by the semiconductor layer and free carriers are generated due to photoconductive effect and collected by the electric field across the semiconductor layer through biasing the contacts to leverage the potential energy of electrons in the conduction band and holes in the valence band and would then contribute to the photocurrent.
In terms of photosensing materials, amorphous silicon has been used in most current MSM PDs, however, the wavelength selectivity of such PDs is primarily governed by the thickness of the semiconductor layer. As a result, in current MSM PDs, the leakage current level still does not meet the stringent requirement for high-sensitive and low-noise indirect conversion X-ray imaging applications.
Therefore, there is provided a novel lateral radiation detector for using in a charge sensing system.